Our earlier U.K. patent 1520030 describes a device of this kind which uses a bifilar choke connected between the heating element and the vehicle d.c. power supply. The choke presents low impedance paths to the relatively large current flow required to power the heating element, and high impedance paths to radio signals.
In practice, when used with a heated window typically consuming 17 A at 12 V for heating purposes, two bifilar chokes may be used, one of relatively small inductance, about 1 .mu.H, which is effective in the vhf range (from 50 MHz to 400 MHz), and a second of much larger value, ideally greater than 10 mH, which is effective over the lowest frequency range of operation, typically down to 150 kHz. Around this lowest frequency, the impedance of the usual heater antenna with respect to the vehicle bodywork closely approaches that of its capacitance, in the region of 80 pF, with a reactance of about 12K.OMEGA. at 150 kHz. The use of the double-wound bifilar configuration is of key importance for this latter choke: since the d.c. magnetisation of the two windings will cancel, a closed magnetic core, such as a ferrite pot-core, may be used without saturation occurring. The required inductance can therefore be achieved with a relatively small number of turns of the thick conductor required to carry the d.c. operating current (say 1.6 mm diameter for 17 A) with a pot-core of acceptable bulk.
With this arrangement, the impedance of the choke is high compared with that of the antenna at the signal frequency whereby a strong radio signal having good signal to noise ratio can be separated.
However, even with the advantage of the double-wound bifilar configuration the low frequency range choke remains a relatively large and expensive component. In practice therefore a compromise value of inductance, say about 1 mH, is employed, for example with 91/2 double turns on a 30 mm diameter core, and this results in some loss of performance at the lowest frequencies. Also, there can be cases where the size or cost of even such a compromise choke may be disadvantageous.
An object of the present invention is to provide an effective signal separating device with which it is possible to achieve good performance at low frequencies with a choke configuration of relatively small size.